Recent work in our laboratory has demonstrated that certain drugs may be attached to well-defined "carrier" molecules and still retain the ability to bind to the receptor site and effect biological activity. This synthetic strategy for the attachment of drugs to carriers is termed the "functionalized congener" approach. The "carrier" molecule may be many times larger than the parent drug; indeed there is practically no maximum size limitation for a fully potent analog. Unlike the prodrug approach or the immobilization of drugs for slow release, the "functionalized congener" approach is designed to produce analogs for which no metabolic cleavage step is necessary for activation. Moreover, the attachment of the drug to a "carrier" such as a peptide may result in the enhanced affinity at an extracellular receptor site and an improvement in the pharmacological profile of the parent drug. The A1-adenosine receptor from bovine brain was purified to homogeneity by affinity chromatography using an immobilized xanthine functionalized congener. This strategy is being employed in the design of new agonists and antagonists at muscarinic acetylcholine receptors and at P2 purinergic receptors. These ligands are synthesized and characterized and then screened for potency and selectivity in binding assays and in functional assays. We have probed sites for attachment of "carrier" molecules to pirenzepine, an M1-muscarinic antagonist that is used in the treatment of gastric ulcers. A particular site for amino-alkyl chain derivatization that enhances biological potency was found.